[C# Guide] Explain that `null` is a constant pattern, and won't match any type

[BillWagner]

Our samples on pattern matching are correct, and we mention that null doesn't match any type in an is or case pattern matching expression.

this topic should add a paragraph or two explaining the behavior for null. We can expand with the justifications:

1. The existing is expression always returned false for null. It is more consistent for the new extensions to have the same semantics on null.
2. Having null never match a type means that developers do not need the explicit null check after matching a pattern. That is a factor in favor of this design.
3. If non-nullable types are added to the language, this decision will make everyone happy.

[cartermp]

What about the default case?

var foo = SomethingWhichReturnsNull();
switch (foo)
{
    case Bar: ...
    case Baz: ...
    case Qux: ...
    default: ... // I'd expect code to execute here
}

In general this seems like the problem we have in F# when we pattern match on objects and test for the subtype, but the resultant type could still be null. You can't be exhaustive with this, but it still results in the need to have a "gotta catch the null" bucket if you don't have a preceding check. Well, either that or a bug in the code. It's a pretty strong motivation for different types than those which can be implicitly null.

[BillWagner]

There might be different semantics. We should discuss to make sure we point out the distinctions.

Consider this (very contrived) C#:

var s = MethodWhichReturnsStringOrNull();
switch (s)
{
    case string str: 
        Console.WriteLine($"{str} is not null");
        break;
    case null:
        Console.WriteLine("method returned null");
    case default:
        Console.WriteLine("won't get here in this example.");
}